Tool for forming internal screw-threads and method of making the same

ABSTRACT

A tool for forming internal screw-threads comprising a base material, a first screw-thread of a uniform cross-section wound spirally in convolutions on the outer peripheral surface of said base material, a number of cutout portions disposed at intervals as required along the entire length of said first screw-thread, and a second screw-thread of a uniform cross-section differing from the cross-section of said first screw-thread and formed in the space of said cutout portions. Either said first screw-thread or said second screw-thread coincide in its cross-section with an internal screw-thread to be formed by said tool, and said two screw-threads are provided with a number of relief portions without reducing the mechanical strength of the screw-threads, so that the single operational length of the tool is minimized and the overall operational length thereof is maximized.

United States Patent Ohmoto [45] July 3,1973

[75] Inventor: Seiii Ohmoto, Tokyo, Japan [73] Assignee: Seiji Ohmoto and Kabushiki Kaisha Nagoya Rashi Seisakusho,

Nakamura-ku, Nagoya, Japan 22] Filed: Dec. 10, 1970 211 Appl. No.: 96,753

Primary Examiner-Charles W. Lanham Assistant Examiner-E. M. Combs Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT A tool for forming internal screw-threads comprising a base material, a first screw-thread of a uniform crosssection wound spirally in convolutions on the outer peripheral surface of said base material, a number of cutout portions disposed at intervals as required along the entire length of said first screw-thread, and a second screw-thread of a uniform cross-section differing from the cross-section of said first screw-thread and formed in the space of said cutout portions. Either said first screw-thread or said second screw-thread coincide in its cross-section with an internal screw-thread to be formed by said tool, and said two screw-threads are provided with a number of relief portions without reducing the mechanical strength of the screw-threads, so that the single operational length of the tool is minimized and the overall operational length thereof is maximized.

3 Claims, 15 Drawing Figures PAIENIEDJUL a ma 5.742.541

MEI! 0f 2 IO IO 20 2 ew/mad F l 6. 58 F 6, 5c INVENTOR ATTORNEY! TOOL FOR FORMING INTERNAL SCREW-THREADS AND METHOD OF MAKING THE SAME This invention relates to .a tool for forming, internal screw-threads in a hole formed beforehand in a workpiece of metal or other material easy to work on by dint of its plasticity.

An object of this invention is to provide a tool which can reduce a torque produced when internal screwthreads are formed by the tool without reducing the strength of the internal screw-threads to be formed by utilizing the plasticity of material.

Generally, a tool by forming internal screw threads refers to a piece of metal having a continuous spiral groove or thread formed on the outer peripheral surface of a base material and arranged in convolutions intimately disposed adjacent one another. In such a tool, a base material is either cylindrical or conical in shape and a screw-thread generally refers to an elongated groove substantially equilaterally triangular in section. The tool according to this invention is characterized by having a threaded portion which is not uniform in section and has two different sectional shapes, such threaded portion comprising convolutions of spiral threads of two different sectional shapes disposed alternately at intervals as required and arranged in close proximity to each other. This invention provides 'an improvement in the threaded portion of a tool disclosed in Japanese Pat. No. 536,962 entitled Tool for Forming Internal Screw-Threads and Method of Making the Same, such improvement residing in providing a separate screw-thread in a space between the projections for reinforcing the tool without reducing the effectiveness of the projections in operation. It will therefore be seen that in the present invention the material for the tool may be either cylindrical or non-cylindrical in section, and a cylindrical or non-cylindrical material may be tapered suitably. However, the shape and configuration of the threaded portion of the tool according to this invention is mainly based on the use of a material of cylindrical shape and uniform diameter.

Additional objects as well as features and advantages of this invention will become evident from the description set forth hereinafter when considered in conjunction with the accompanying drawings, in which:

FIG. I is a fragmentary schematic perspective view of the screw-threads of the tool according to this invention; 7

FIGS. 2 and 3 are a plan view and a front view respectively of the screw-threads of FIG. 1;

FIG. 4 is a sectional view taken along the line X-X of FIG. 2 and seen in the direction of arrows;

FIGS. 4A and 4B are views similar to FIG. 4 but showing different embodiments; I i

FIGS. 5, 5A, 5B and 5C are views similar to 'FIG. 4 but showing an embodiment which has a planar portion at the crest of one screw-thread;

FIG. 6 is a schematic sectional view of the tool according to this invention which is normal to the axis of the root of the tool; and

FIGS. 7 and 7A and FIGS. 8 and 8A are fragmentary front views of the screw-threads in explanation of the shapes of the cutout portion and remainder portion provided in one screw-thread. I

In the present invention, a threaded portion comprises a screw-thread 10 of a definite cross-section which is formed with cutout portions of suitable size and shape disposed at intervals as required throughout the length of the screw-thread, and the other screwthread 20 of a definite cross-section which fills the space of the cutout portions of the first screw-thread. Stated differently, the threaded portion 30 of the tool according to this invention may be said to be of such shape that portions of one screw-thread 10 of a definite cross-section are arranged at intervals as required on the other screw-thread 20 of a definite cross-section. Generally, one screw-thread has a cross-sectional shape which conforms to that of an internal screwthread to be formed by the tool according to this invention, and the other screw-thread has a height and a sectional shape which differ from those of said one screwthread.

As aforementioned, the threaded portion 30 of the tool according to this invention comprises a regular screw-thread having a cross-sectional shape which conforms to that of an internal screw-thread to be formed by the tool according to this invention, and the other screw-thread having a height and a sectional shape which differ from those of said one screw-thread, such two screw-threads being disposed at intervals as required and arranged in close proximity to each other. The threaded portion of the tool according to this in vention is of such shape that it has a number of reliefs provided on the circumferential surface along the threaded portion 30. In external appearance, the

threaded portion appears to consist ofa combination of the two screw-threads l0 and 20. Therefore, the threaded portion of the tool according to' this invention will hereinafter be referred to as a composite screwthread". The composite screw-thread is no different from its equivalent described in Japanese Pat. No. 536,962 in that it relies less on its cutting action than on its action to cause plastic deformation to take place in a workpiece, the latter action being predominantly effective in accomplishing the end of forming internal screw-threads by the tool.

It will be evident that the composite screw-thread of the tool according to this invention can have forms of many different varieties by suitably combining two thread-screws of different cross-sectional shapes. However, when the objects of the invention or the reduction of a torque produced when internal screw-threads are formed by the tool of this invention and the ease with which the tool can be made by form rolling are considered, the tool according to this invention is naturally limited to a certain number of forms. Some of the forms of the tool which are considered to be capable of accomplishing the aforementioned objects of the invention satisfactorily and which have proved experimentally to have effect will be successively described as embodiments of the present invention. For the sake of convenience, the embodiments will refer to a tool with composite screw-threads having a faceangle of 60.

The threaded portion 30 shown in FIGS. 2, 3 and 4 may be said to be a composite screw of the typical shape. The composite screw comprises the screwthread 10 of a cross-sectional shape with a face angle of 60 which is formed throughout its length with V- shaped cutout portions 11 of a suitable size disposed at intervals as required, the bottoms of such cutout portions 11 being flush with a root 12 of the screw-thread I0 and the remainder portions or flanks I3 of-the screw-thread 10 each have a crest 14 which is of an inverted V-shape. It is to be understood that the cutout portions 11 are not limited to the V-shape, that they may be of any other suitable shape as desired, and that they have a depth which varies gradually in the direction of the screw-thread. In making the tool of this invention by form rolling, an embodiment in which the cutout portions are of V-shape and a definite depth and arranged as much geometrically as possible is easier to produce by plastic working than other embodiments. The difficulty which is experienced in making the tool by form rolling will increase if an inclined surface 15 of each cutout portion 11 of the V-shape becomes increasingly nearer to perpendicular with respect to the root 12 of the screw-thread 10.

The reason why the remainder portions 13 of the screw-thread are of an inverted V-shape is that this shape can have the effects of wedging a workpiece and reducing a torque in the initial stages of internal screwthread forming in the workpiece. Preferably, the inclined surface 15 of the composite screw-thread is curved as seen in the tooth profile curve of a gear (See FIG. 6) rather than straight. Also, it is preferable to arrange the V-shaped cutout portions 11 helically at a suitable angle with respect to the longitudinal center axis of the tool. The tool with this arrangement iseasier to make by form rolling than the tool in which the V- shaped cutout portions are arranged parallel to the axis.

If the area of a planar portion 16 of the crest 14 of each remainder portion 13 of the screw-thread 10 is increased as shown in FIGS. 8 and 8A, then the tensile strength of the screw after the tool is threadably fastened to the internal screw-thread formed is increased, but a torque produced when internal screw-threads are formed by using this tool is increased. On the other hand, if its area is reduced till it becomes substantially a point as shown in FIGS. 7 and 7A, then the torque is reduced, but the tensile strength of the screw is also reduced. Thus, it is important to suitably select the shape and configuration of the remainder portions 13 of the screw-thread 10 depending on the size and pitch of the internal screw-thread and the purpose for fastening the tool to the internal screw-thread. The remainder portions 13 shown in FIGS. 7A and 8A are spaced apart from each other by a planar portion 17 formed in the root 12 by spreading transversely the V-shaped cutout portions 11. The greater the area of the planar portions 17 of the cutout portions 11, the greater is the spacing between the remainder portions 13. Thus, the cutting portions in one convolution of the screw-threads of the composite screw or the complete screw-threads are reduced in number.

In this way, a space in the first screw-thread 10 produced by the provision of the cutout portions 11 of the V-shape is filled and reinforced by portions of the second screw-thread 20 of a definite cross-section which resembles the regular screw-thread in shape and is different in height from the screw-thread 10. Accordingly, the difference in height between the two different screw-threads in the cutout portions 11 manifests itself as offset portions 31 which are either planar or curved. The offset portions 31 have edges 32 which inevitably perform a cutting action when internal screw-threads are formed by this tool. However, when the actions of the tool are' considered as a whole, the tool performs very little cutting action which is of little importance and therefore negligible in comparison with the action of the tool to produce elastic deformation in a workpiece, with only a minute amount of chips being produced in the workpiece.

The cutting action of the composite screw-thread, though it is not an important function, has an effect of improving the finishes of the screw surface of an internal screw-thread to be formed by the tool. The chips of minute grain size which are produced by the cutting action are accommodated by spaces 33 remaining in the cutout portions 1 1 and fill part of the spaces 33, so that the composite screw can be more intimately fastened to the internal screw-thread than otherwise. A linear portion 34 which is at right angles to the threaded portion 30 is formed at the lowermost portion of each cutout portion 11.

In order to facilitate the production of the tool by reducing as much as possible the offset portions 31 which are either planar or curved and the linear portions 34, it appears that the screw-threads 10 and 20 of the cross-sectional shape shown in FIGS. 4A and 4B can achieve better results. In these embodiments, the root of the second screw-thread 20 has a width which is equal to that of the root of the first screw-thread l0 and the face angles of the screw-threads are increased, thereby reducing the height of the crests of screwthreads. It should be noted that in the embodiment shown in FIG. 4B the second screw-thread 20 has inclined surfaces 21 which coalesce with inclined surfaces 18 of the first screw-thread 10 in their lower parts.

If is desired that the composite screw of the tool according to this invention be fastened to the internal screw-thread, the second screw-thread 20 which fills the cutout portions 1 1 of the first screw-thread 10 may be shaped such that the center of the second screwthread 20 is displaced either to right or left with respect to the center axis of the first screw-thread 10. When, this is the case, the direction in which the displacement of the center axis of the second screw-thread 20 takes place is decided depending on the direction of loosening of the screw with respect to the axis of the tool. Generally, the displacement of the center axis is effected such that one inclined surface of the screwthread 10 coalesces with one inclined surfaces of the screw-thread 20.

Generally, a planar surface is provided at the crest of the first screw-thread 10 by cutting off about oneeighth the screw-thread in height according to the standards of screws. Therefore, it is possible to align the crest of the first screw-thread 10 with the crest of the second screw-thread 20 at the same height. FIGS. 5, 5A, 5B and 5C show embodiments of the type described.

The shape of the threaded portion 30 of these embodiments of the composite screw-thread may be suitably selected from among the group of shapes described above depending on which of the objects of the invention or the reduction of a torque produced when internal screw-threads are formed, the maintenance of tensile strength if the tool is fastened to the internal screw-threads, and the ease with which the tool is made by form rolling the particular composite screw .is particularly intended to accomplish. Also, by using other shapes which can readily be thought of from these shapes for the threaded portion of the tool according to this invention, the overall objects of the invention can be accomplished although the result achieved in one aspect of the invention might not be as great as it should be.

The tool having the composite screw-thread generally performs the functions of forming an internal screw'thread. A material used for making such tool having the composite screw-thread is generally hardened by heat-treatment.

From the foregoing description, it will be appreciated that the tool having the composite screw-thread according to this invention comprises a threaded portion 30 which combines the first screw-thread with the second screw-thread 20, such screw-threads being uniform in cross-section. The composite screw-thread is shaped such that the root thereof is generally of uniform diameter in every portion and a number of reliefs are provided on the circumferential surface without reducing in any way the strength of the screw-threads themselves. The tool having the composite screwthread constructed as aforementioned offers many advantages which have hitherto been unattainable in conventional tools. It is capable of reducing the torque produced when internal screw-threads are formed thereby. By form rolling it is possible to make the tool of this invention efficiently on a mass production basis by a single operation form rolling with a blank of a circular cross-section.

While some embodiments of the invention have been shown and described above, it is to be understood that the composite screw-thread and the die for producing the same by form rolling according to this invention are not limited to the specific forms, constructions and processes shown and described herein, and that many changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A tool for forming internal screw-threads comprising a base material and a composite screw-thread having a first thread-screw of uniform cross-sectional shape formed spirally on the outer peripheral surface of said base material with uniform thread pitch, and a second thread-screw of uniform cross-sectional shape different from that of said first screw-thread formed in the same direction as said first screw-thread, said first screw-thread being formed at suitable spacing through out substantially the full length thereof with a number of cutouts substantially in the form of a letter V having a depth such that the cutouts reach substantially the root of the screw-thread, said cutouts formed in said first screw-thread providing therein discrete spaces each having therein one of a number of portions into which said second screw-thread is suitably divided, said one portion of said second screw-thread substantially filling the major portion of each of said discrete spaces in said first screw-thread so as to provide a number of relief portions on the flanks of the first screw-thread,

the spaces in the body of the first screw-thread being arranged helically at a suitable angle with respect to the center axis of the tool, so that said first screw-thread and said second screw-thread divide each other into suitable lengths disposed close to each other at regular intervals. 7

2. The tool as defined in claim 1 in which the root of the second screw-thread has a width which is equal to that of the root of the first screw-thread.

3. The tool as defined in claim 1 in which the root of the composite screw-thread is of uniform width. 

1. A tool for forming internal screw-threads comprising a base material and a composite screw-thread having a first thread-screw of uniform cross-sectional shape formed spirally on the outer peripheral surface of said base material with uniform thread pitch, and a second thread-screw of uniform cross-sectional shape different from that of said first screw-thread formed in the same direction as said first screw-thread, said first screw-thread being formed at suitable spacing throughout substantially the full length thereof with a number of cutouts substantially in the form of a letter V having a depth such that the cutouts reach substantially the root of the screw-thread, said cutouts formed in said first screw-thread providing therein discrete spaces each having therein one of a number of portions into which said second screw-thread is suitably divided, said one portion of said second screw-thread substantially filling the major portion of each of said discrete spaces in said first screw-thread so as to provide a number of relief portions on the flanks of the first screwthread, the spaces in the body of the first screw-thread being arranged helically at a suitable angle with respect to the center axis of the tool, so that said first screw-thread and said second screw-thread divide each other into suitable lengths disposed close to each other at regular intervals.
 2. The tool as defined in claim 1 in which the root of the second screw-thread has a width which is equal to that of the root of the first screw-thread.
 3. The tool as defined in claim 1 in which the root of the composite screw-thread is of uniform width. 